Until the 1980's, controllers using surveillance data, derived from the ATCRBS and Mode S systems, tracked aircraft and provided separation assurance and, when necessary, collision avoidance warnings and maneuver instructions. With the initiation of TCAS, collision avoidance, for equipped aircraft, could now be performed independently by the pilot. The TCAS system leveraged the FAA surveillance system so that tracking functionality was derived from a system that was independent of the navigation VOR/DME system.
With the advent of augmented GPS, ATC navigation could be made extremely accurate everywhere and at a reduced cost to the FAA. It was only natural that the concept of leveraging GPS and/or Galileo (and/or equivalent satellite navigation system) for tracking, separation assurance, and collision avoidance be explored. Such a system is the automatic dependendent surveillance ADS-B.
The following quotes from the ADS-B web site describe its functions and its benefits. Standards for Automatic Dependent Surveillance—“Broadcast (ADS-B) is currently being developed jointly by the FAA and industry through RTCA Inc. Special Committee 186 (SC-186). The concept is simple: Aircraft (or other vehicles or obstacles) will broadcast a message on a regular basis, which includes their position (such as latitude, longitude and altitude), velocity, and possibly other information. Other aircraft or systems can receive this information for use in a wide variety of applications. Current surveillance systems must measure vehicle position, while ADS-B based systems will simply receive accurate position reports broadcast by the vehicles.”
“In comparison with today's surveillance system, ADS-B's accuracy is now determined by the accuracy of the navigation system, not measurement errors. The accuracy is unaffected by the range to the aircraft. With the radar, detecting aircraft velocity changes requires tracking the received data. Changes can only be detected over a period of several position updates. With ADS-B, velocity changes are broadcast almost instantaneously as part of the State Vector report. These improvements in surveillance accuracy can be used to support a wide variety of applications and increase airport and airspace capacity while also improving safety.”
The use of augmented GPS/Galilco for navigation, separation assurance and collision avoidance takes advantage of the three dimensional high accuracy that satellite based positioning can provide. This improved accuracy can indeed allow for closer spacing (increased capacity). Given that all tracking is derived from aircraft instrumentation, then the potential is that nearly all ATC can be performed in the cockpit, which would eliminate the need for most controllers and active surveillance systems. The result being a higher capacity system at a significantly lower cost when referenced to today's system. This is the potential and significance of ADS-B.
The problem is today's surveillance system is not safe and ADS-B will make it even less safe. A saboteur can obtain accurate position locations of aircraft today by using multilateration on Aircraft Mode S/ATCRBS replies to obtain range, position and tracking information of aircraft in the TRACON airspace. Thus a small missile can be GPS navigated to an accurately tracked target.
Multilateration is a technique whereby one measures the time of arrival from four or more widely separated receivers, and takes the difference in the time of arrivals to determine the position of the transmitting aircraft. If the signal is strong and readable and the geometry is good, than accurate position measurements can be made. By good geometry is meant that the transmitting aircraft is roughly flying to positions which are within the area set up by the ground receivers. The small missile threat is limited to the TRACON area since range and altitude are constrained by the missile size.
Today's air traffic control surveillance system is comprised of the radar beacon system (ATCRBS) and Mode S (discrete address beacon system). As a backup system the FAA has developed and aircraft are equipped with the Traffic Alert and Collision Avoidance System (TCAS). This system uses data received from airborne transponders responding to their ATCRBS/Mode S interrogations. The TCAS receiver then performs a two-way range measurement, reads the ATCRBS message to determine altitude and aircraft identity and finally makes a rough bearing measurement. The range measurement is performed by taking the difference between the times of arrival of the reply to the time of transmission of the ATCRB interrogation.
With respect to the multilateration threat, a saboteur need only purchase 4 TCAS receivers which are modified to determine time of arrival and possibly altitude for both ATCRBS or Mode S replies to TRACON ATCRB/Mode S interrogations, use a GPS/WAAS time transfer unit at each site to insure relative timing accuracy measurements between receiver sites and a TCAS like algorithm for determining tracks. Thus there is some investment and engineering that has to be performed by the terrorist to exercise this threat.
ADS-B is far worse. A saboteur need have only one aircraft ADS-B commercial radio. This enables him to receiver ADS-B messages that provide position and aircraft identity information and to track all aircraft in the vicinity. That is what commercial ADS-B avionic equipment is designed to do. Small modifications allow the saboteur to extract this information to provide continuous track information. Thus one can use small guided missiles which navigate accurately using GPS and which track multiple A/C accurately using GPS. No visual citing required.
How accurate is ADS-B? In addition to GPS, aircraft utilize the wide area GPS augmented system (WAAS) to improve the system. The following is a quote from the FAA web site. “The WAAS message improves the accuracy, availability and integrity (safety) of GPS-derived position information. Using WAAS, GPS signal accuracy is improved from 20 meters to approximately 1.5-2 meters in both the horizontal and vertical dimensions.” In the future with the next generation GPS and with the use of GPS together with Galileo the accuracy uncertainty will reduce to under 1 meter.
Encryption of ATC surveillance replies would deny position data to the unauthorized.
Can ADS-B messages be encrypted? Because of the way ADS-B works all aircraft within a given geographic area would have to use the same security codes since all are transmitting and all are receiving each other's ADS-B transmissions. This would be a group encryption so that every IFR pilot (and possibly General aviation pilots) within a region could obtain this information. If one pilot were a terrorist he could relay it to another terrorist on the ground. The group encryption requirement would thus be ineffectual no matter what the update rate was for changing the group encryption code. As a result message security cannot be achieved with ADS-B.
In summary, the surveillance system today can be used by terrorists to inflict great damage in the TRACON airspace. The transition to ADS-B increases the threat significantly, by increases the accuracy of target tracking and substantially reduces the resources needed to carry out a multi missile attack.
The objective of this invention is to provide security, to the surveillance system within the TRACON airspace, against terrorist small missile attacks.
There are a number of strategies for implementing a more secure ADS system. These are defined as ADS-S systems. The selection of the system will be a function of the cost of implementation, the level of security and the associated resources required by the saboteur to counter the security technique. Thus the goal is to implement sufficient security of the ADS-S system so that it is unrealistic for the terrorist to counter the secure system. Three options are given for implementing a secure system. The first (option A) provides only message security. The second and third options ensure that messages cannot be read and multilateration cannot provide the terrorist aircraft tracks.
The invention assures that messages transmitted to/from an aircraft can only be read by the addressed user aircraft and by the ground ATC system. Surveillance messages can be made secure with authentication and/or encryption. This insures that a WAAS based terrorist GPS tracking system cannot be achieved.
The invention, in options B and C, assures that transmissions from the aircraft to the ground need to be designed so that aircraft cannot be tracked by using multilateration ranging measurements and a set of such measurements to form accurate aircraft tracks.
Transmissions from the aircraft have to utilize techniques which do not allow successful ranging and/or tracking. One such technique, as demonstrated in this invention utilizes a hybrid FDMA system with pseudo random (PN) codes which spread the signal over a wide bandwidth relative to the information bandwidth. Thus there are many PN chips that are transmitted within one information or framing bit. The PN code is made up of ±1 chip values so that the average sum value of all chips in a framing bit is about zero. As will be shown this can be achieved when a long code is used to spread the signal under the noise. A second element in ensuring that the ADS-S signal cannot be ranged on is to design the system so that it is transmitted under the noise as seen by the saboteur's terminal. Several design options for achieving this are presented.
To ensure security, each aircraft, at the start of its flight, is given an identity code, an encryption code and a spread spectrum code (options B & C). This information can be transmitted within the ATC system via secure terrestrial networks. Any or all of these codes can be changed dynamically, via commands from the ground ADS-S TRACON terminals. To achieve a highly secure surveillance system, A/C cannot squitter (short transmission burst containing ADS information) their location.
The Enroute system utilizes the ADS-B system. The invention is so designed that ADS-B in the Enroute airspace and ADS-S in the TRACON airspace do not cause mutual interference to one another.
The system is so designed that ATCRBS/Mode S operating with ADS-S, in the same TRACON airspace, does not cause mutual interference to one another. This design is necessary to insure a transparent transition from ATCRBS/Mode S to ADS-S.
The ADS-S system is designed with a high data rate ground-air (uplink) capability in the multiple megabit range.
The system is designed to support traditional centralized ATC and with an option for a hybrid distributed and centralized ATC system within the TRACON airspace.
The invention provides three options for a secure surveillance backup system, namely:                1. Using the ADS-S terminal to perform 2 way ranging, bearing determination using monopulse detection and receiving the barometric altitude reading in the ADS formatted message.        2. Transmitting a navigation backup system position determination via an ADS-S formatted message to the ground terminal.        3. Knowing the time an aircraft is interrogated by the ADS-S terminal means that only an additional two terminals are needed to multilaterate.        
If multilateration is designed as the surveillance backup system it can also be used as an anti spoofing system even when the ADS-S system is operating normally.